[Modification Method of Resin Surface Layer]
As modification methods of molded resin articles, for example, methods of irradiating the articles with electromagnetic waves such as an X ray or a gamma ray are known. However, modification methods of modifying “a surface layer” alone extending to a certain depth from the surface of each molded resin article but not modifying an inside portion of the molded resin article are extremely limited, and these methods are also accompanied by many restrictions.
For example, a method of hitting an ion beam of a metal or an organic compound on a resin surface is known. However, some restrictions are placed on this method of hitting the ion beam, and for example, it is difficult to uniformly treat the whole of a resin film having a large area, it is also difficult to uniformly treat the whole surface of the molded resin article having a complicated shape, and the organic compounds usable as the ion beam are limited.
Moreover, for example, there is known a method (laser ablation method) of irradiating the resin surface with a laser pulse of a high power density for a short time to modify the resin surface. However, similarly on the laser ablation method, some restrictions are placed, and for example, it is difficult to uniformly treat the entire large-area resin film, and it is also difficult to uniformly treat the entire surface of the molded resin article having a complicated shape.
Furthermore, for example, there is known a method of subjecting the resin surface to a plasma treatment under atmospheric pressure to oxidize/modify the surface, but some restrictions are placed on this method, and for example, the method cannot be applied to a surface which is hard to be oxidized, and the effect does not last for a long time.
When a treatment agent for modifying the resin surface layer is a liquid, the treatment agent can be applied onto the surface of the molded resin article. On the other hand, when it is a solid, it can be applied thereonto in the form of a solution obtained by using an appropriate solvent, or the molded resin article can be immersed in the solution. However, in such a wet type method, the smoothness or the fine structure of the resin surface is easily lost, and washing is further necessary so that the excess treatment agent may not remain on the resin surface. During this washing, the effect of the surface treatment tends to be impaired. Moreover, when the solvent is used, some restrictions are placed on the method, and for example, there is an inconvenience that the resin surface swells owing to the solvent.
Moreover, there has also widely been used a method of mixing/kneading the treatment agent for modifying the resin surface layer with the whole resin, followed by molding. However, in this method, the treatment agent which is usually necessary only for the surface layer needs to be mixed with the whole resin, and as a result, the treatment agent mixed with the whole resin adversely affects the physical properties of the resin sometimes. On the other hand, in order to prevent the treatment agent mixed with the whole resin from adversely affecting the physical properties of the resin, some inconveniences are present. For example, the treatment agent cannot be added in a necessary and sufficient amount, and kinds of treatment agents which can be mixed with the whole resin are restricted.
[Coloring Method of the Surface of Molded Resin Article]
As a coloring method of the molded resin article, there are broadly used, for example, after kneading a dye or a pigment with a resin, (1) a method of injection-molding the resin to obtain a colored molded resin article, (2) a method of stretching the resin to obtain a colored film-like article, and (3) a method of spinning the resin to obtain a fiber-like article.
On the other hand, as a method of coloring the surface only of the molded resin article without changing its internal composition, various coating methods and printing methods are broadly used.
Except for an especial method of depositing a dyestuff alone penetrable into the resin surface layer on the surface of the molded resin article by the coating method or the printing method and then allowing the dyestuff to penetrate thereinto, in the case of the usual coating method or printing method, a layer comprising a mixture of a binder resin and a dye or a pigment is formed on the resin surface. Strictly speaking, such a coloring method means not “coloring of the surface itself”, but “adding and attaching a colored film to the surface”. Therefore, if the resin surface is finely worked, there is a problem that the finely worked portion is buried with the added film. Moreover, when the coating method is used, the smoothness or the like on the surface of the molded resin article tends to be impaired owing to dissolution or swelling with the coating solvent. Therefore, it is required to avoid the dissolution or the swelling of the resin surface layer, but any appropriate coating solvent cannot be found. In consequence, it is difficult to apply the coating method on occasion. For example, when a film comprising a dyestuff and poly(methyl methacrylate) is formed on the surface of the molded resin article of poly(methyl methacrylate) by the coating method in which a solvent is used, the solvent for dissolving poly(methyl methacrylate) necessarily erodes the surface of the molded resin article of poly(methyl methacrylate), and hence, it is not easy to obtain the smooth coated surface.
In the case of depositing a coating film or a print film containing a sublimable dyestuff on the resin surface to color the surface, the following problems further occur.
(i) In applying or printing a composition comprising the sublimable dyestuff and the resin, the sublimable dyestuff sublimates in the course of a process sometimes. Therefore, control for maintaining the composition and control for uniforming the composition are not easy.
(ii) It is extremely difficult that the sublimable dyestuff alone is left as it is and other volatile impurities are removed. Therefore, it is not easy to prepare the coating film or the print film which does not contain the volatile impurities other than the sublimable dyestuff.
Besides the method of forming the coating film or the print film on the resin surface, a deposited film of the sublimable dyestuff may be formed to color the molded resin article. In a usual vapor deposition method, temperature of a vapor deposition source is set to be higher than temperature of a substrate subjected to film formation, and the vapor of the organic compound which flies from the vapor deposition source is deposited on the surface of the substrate subjected to film formation with lower temperature. In the film forming method under such non-equilibrium conditions, film thickness control is performed mainly by mechanical means. That is, a mask or a slit is placed in a space extending to the substrate to be filmed thereon from the vapor deposition source, and the concentration of the vapor of the organic compound is then controlled in a spatial manner, or the substrate to be filmed thereon is rotated, whereby the thickness of the deposited film can be uniformed. However, when a vapor pressure of an organic compound (dyestuff) is high and sublimation easily occurs, it is not easy to obtain a uniform thickness of the deposited film only with such mechanical means.
As an image printing method using sublimation, a sublimable dye transfer method is known. That is, the sublimable dyestuff formed into the film on the surface of a sublimable dye transfer ribbon or sheet is heated with a heating head, sublimated, and deposited or allowed to penetrate into a transfer layer (thin film of thermoplastic resin) of a sheet surface disposed in the vicinity in the method. In principle, the method is equivalent to the vapor deposition method, and image density is controlled by a heating area and heating amount in the heating head. Therefore, this is suitable for a purpose of printing an image having a fine gradation in principle of “pointillism”, but unsuitable for coloring the resin surface of a large area with a uniform density. Moreover, it is unsuitable for coloring the molded resin article having a complicated surface shape.